Top level enums are not handled

[clux]

A rare case that we haven't seen, but should support because kube supports it, and it makes unit testing easier (no need to wrap enums in structs to check their output). Came out of #100

There is a test for this top_level_enum_with_integers which is currently #[ignore]d because of this bug.

We should handle enum members at the top level like we do for struct members.